Process for preparing drawn polyamide films

ABSTRACT

POLYAMIDE FILM IS PROCESSED BY ADDING 1 TO 6% BY WEIGHT WATER, PREHEATING AND THEN BIAXIALLY DRAWING AT A TEMPERATURE AT LEAST 5*C. HIGHER THAN THE PREHEATING TEMPERATURE. DURING PREHEATING A SUBSTANTIAL DIFFERENCE IN WATER CONTENT BETWEEN THE INTERIOR AND SURFACE OF THE FILM IS CREATED. THE FILMS MADE BY THIS PROCESS HAVE A RELATIVELY LOW FRICTION COEFFICIENT.

PROCESS FOR PREPARINGDRAWN POLYAMIDE FILMS AND FILMS PRODUCED THEREBYFiled July 23 1968 Feb. 2,1971 UTS'UO K'U A ETAL 3,560,606

.LNE'IOL-JdEIOO Nouomj United States Patent Office 3,560,606 PROCESS FORPREPARING DRAWN POLYAMIDE FILMS Mutsuo Kuga and Kayomon Kyo, Kyoto-shi,Takeshi Mashimo, Uji-shi, Wakuo Matsumura, Hirakata-shi, and HiroshiKayama, Uji-shi, Japan, assignors to Nippon Rayon Kabushiki Kaisha(Nippon Rayon Co. Ltd.), Kyoto-fu, Japan, a body corporate of JapanFiled July 23, 1968, Ser. No. 746,889 Claims priority, applicationJapan, July 25, 1967, 42/47,768 Int. Cl. B29c 17/02 U.S. Cl. 264-289 10Claims ABSTRACT OF THE DISCLOSURE Polyamide film is processed by adding1 to 6% by weight water, preheating and then biaxially drawing at atemperature at least C. higher than the preheating temperature. Duringpreheating a substantial difference in water content between theinterior and surface of the film is created. The films made by thisprocess have a relatively low friction coefficient.

Processes for the production of polyamide films involving thesimultaneous longitudinal and transverse drawing of a substantiallyamorphous polyamide film has recently been proposed. In such processes,drawing is effected in both the longitudinal and transverse directionsat a draw speed of from 6,000 to 100,000 percent/min. and at atemperature within the range of from 70 C. to 180 C., which is at least35 C. below the melting point of the film. The ratio of the longitudinalto transverse draw speeds is generally from 2:1 to 05:1, and the drawratio from 16:1 to 4:1. The polyamide films obtained according to thesemethods have various advantageous properties such as improved tensilestrength, and optical clarity for general industrial uses. Although theyhave a very even surface, high transparency and beautiful luster, theydo however have several disadvantages in use. Such films, for example,have a high friction coefficient with themselves or with printingmachines when they are processed e.g. for printing, bag making, adhesionand various other purposes.

Moreover, operational efficiency may be impaired due to their highadherent properties. It is an object of the present invention to provideimproved polyamide films having low friction coefficient, and lowadherent properties.

SUMMARY OF INVENTION According to the present invention there isprovided a process for drawing a polyamide film which comprises treatingthe polyamide film with Water, preheating the film to produce adifference of water content between the surface and the interior of thefilm and drawing the film simultaneously in the longitudinal andtransverse direction.

According to the process of the invention it is possible to obtain apolyamide film having uneven reticulated fine projections on itssurface, and a low friction coefiicient. Moreover, blockingcharacteristics are reduced and the processability of the film isimproved. The film obtained also has a uniformity of properties.

According to the present invention, better results may be achieved bydrawing the film simultaneously and biaxially in an atmosphere having ahigher temperature than that of the preheated film. The presentinvention gives particularly advantageous results when drawingconditions are controlled so that the draw speed of the polyamide filmis conveniently 6,000-100,000 percent/ 3,560,606 Patented Feb. 2, 1971draw speed=d [1 X (percent/ t) wherein d is thickness of the drawn film,d is thickness of the film before drawing, and t is the time in minutesnecessary to draw the film.

(2) Ratio of draw speeds can be defined as follows:

longitudinal draw speed ratio of draw speeds= transverse draw speed inwhich the longitudinal draw speed and transverse draw speed showrespectively normal values or mean values of draw speeds in longitudinaland transverse directions from the begin of drawing to an optional pointreached in the drawing process. Draw speeds are measured at the saidpoint.

In other words, when a film having a length of L and width of H isdrawn, the longitudinal draw speed and transverse draw speed can bedefined respectively as follows:

l L 100 percent/t and h /H Xl00 percent/t wherein I and I1, arerespectively the length in the transverse direction and width of thedrawn film, which are measured after 2 minutes from the beginning of thedrawing at an optional point on the film track.

(3) In industrial practice of biaxial and simultaneous drawing, it ispreferred to define the draw ratio as d /d in which d is the thicknessof a drawn film and d is the thickness of said film before drawing.

The film obtained has unevenly reticulated fine projections (or webs) onthe surface and an improved friction coefficient.

The method of giving a film a difference of water content between itssurface and interior is preferably as follows. A film having the desiredwater content is preheated in the shortest possible time to a suitabletemperature in order to achieve a difference of water content betweenthe surface and the interior. It is then drawn simultaneously in thelongitudinal and transverse directions in a drawing zone where thetemperature of the atmosphere is higher than the temperature of thepreheated film, so that the surface of the preheated film is heatedduring drawing to evaporate water from its surface and provide a largedifference of water content between the surface and the interior. Asused herein, the term polyamide films includes films made from linearpolyamides, for example, poly-E-caproamide,poly-hexamethylene-adipamide, poly-hexamethylene-sebacamide,polyll-aminoundecanamide, copolymerized polyamide, etc. Thewater-content of the polyamide film is preferably from 1 to 6% by weightbefore preheating. Preferred amounts of water which are added in thepretreatment of the polyamide films vary according to the types ofpolyamide film and may for example be within the following approximateranges:

Water content (percent Polyamide by weight of film) Poly-e-caproamide1-6 Polyhexamethylene-adipamide 1-6 Polyhexamethylene-sebacamide 1-5Poly-1l-amino-undecanamide 1-3 These values may however vary accordingto the temperature and other conditions under which the film is treated.The relationship between the water content of the polyamide film beforedrawing and the surface properties of the films obtained may be asfollows:

(1) Raw film having too high a water content may give rise to a drawnfilm having uneven reticulated fine projections (webs) on its surfacewhich may grow gradually and simultaneously result in a decrease oftheir heights. The friction coefiicient of the film may increase as thewater content in the raw film increases.

(2) Low water contents of the raw film before drawing may give rise to alower friction coefficient in the drawn film.

According to the present invention, the absorption of water onto the rawfilm is effected prior to the drawing step, and water is added to theundrawn film, for example, by the use of a water bath, water spray, etc.in order to give a uniform water content to the raw film which may bedrawn uniformly. As a result, it is possible to obtain a uniformlyundulating surface on the drawn film.

The water content is preferably uniformly distributed throughout thefilm before drawing, for example, by passing the film through anatmosphere maintained at the correct temperature and humidity. A filmhaving the correct water content is then preheated to a suitabletemperature and drawn simultaneously in the longitudinal and transversedirections whilst maintaining a difference of water content between thesurface and the interior of the film during a drawing process.

Improper heating may give rise to an incorrect difference in watercontent between the surface and the interior of the film. It istherefore necessary to use the shortest possible time to heat the filmto the temperature required for drawing. Suitable heating times andtemperatures may be determined empirically according to types anddimensions of the raw films.

Better results may be obtained by preheating the surface of a raw film,50-500 1. thick, for several seconds, e.g. 0.5 to 30 seconds, in anatmosphere having a temperature from about 70 to 35 C. below the meltingpoint of the film. Furthermore, it is preferred that the temperature ofthe atmosphere in the drawing Zone be maintained at least 5 C. above thetemperature of the preheated film but at a temperature not higher than30 C. below the melting point of the film. This is required in orderthat the surface of the preheated film is heated to evaporate water fromits surface and thereby achieve a larger difference of water contentbetween the surface and the interior of the film. To achieve theimprovements obtainable from the present invention, it is necessary toimpart a large difference in the water content of the film between itssurface and interior at the initial stage of drawing. Generally thesurface of the film at this stage will be substantially dry.

The film may preferably be drawn, at least in part, in a zone where thetemperature of the atmosphere is at least 5 C. above the temperature ofthe preheated film, before the film is drawn such that the ratio betweenthe average thickness of the drawn film and the undrawn film is 0.5:1.The temperature of the atmosphere may be maintained by any suitablemethod, for example, using hot air, infra-red radiation, etc. Afterdrawing of the film, it may be heat-set for example by heating at atemperature not higher than C. below the melting point of the film withthe film maintained under tension such that the transverse directionremains substantially constant.

In use, the most advantageous water content of the raw film may bedetermined by drawing raw films having various water contentssimultaneously in the longitudinal and transverse directions andinspecting the surface of the resultant film. A suitable method ofinspection is by the photographic determination of drawn polyamide film.For example a photographic enlargement (X) has been prepared showing thesurface of a simultaneously longitudinally and transversely drawnpolyamide film prepared from a raw poly-e-caproamide film having athickness of 200a and a water content of 1.2% by weight of film, andobtained by pre-heating the raw film to C. for several seconds, anddrawing simultaneously in the longitudinal and transverse directionswith an elongation of 3X3, with a draw speed of 24,000 percent/min., anda draw ratio of about 1 in an atmosphere having a temperature of 160 C.Uneven reticulated fine projections (webs) which are very closely spacedcan be observed.

Similar photomicrographs have been produced which show the surface ofdrawn films obtained from a similar raw film except that the watercontents of the films were 2% and 3% by weight respectively. As thewater content of the raw film increases, the uneven reticulated fineprojections (webs) become more uniformly distributed. However, unevenmesh-like projections (webs) appear to slowly expand and at the sametime the height of these meshes decreases.

In a film obtained from the similar raw film having a water content of6.0%, it is apparent from the photomicrograph that this film has an evenand smooth surface.

An enlarged photographic appearance of the glycerincoated surface of adrawn poly-e-caproamide film obtained by simultaneous drawing in thelongitudinal and transverse directions of a similar raw film having awater content of 1.2% by weight shows the difference between the innerand outer structures of the drawn film. It may be seen that the innerstructure of the drawn film is similar to that of the transparent filmobtained from a raw film which had a water content of 6.0% by weight.

The densities of both low and high friction films after drying aresimilar and have an approximate value of 1.144. These observations leadone to the conclusion that the inner parts of the film are uniformlydrawn, and that the uneven areas of cracks only occur on the surface.

DRAWING The accompanying drawing graphically represents the frictioncoefficient of the films obtained under the conditions described aboveat various water contents. It is apparent from these figures that thefriction coefficient decreases as the absorbed water content decreases.

The friction coefficient was measured using a commercial instrumentdesigned for measuring the friction coefiicient of textile materials(available from Toyo Sokuki, K.K., Japan) with a slide speed of 0.50cm./sec., a load of grms. the width of the test strip being 0.50 cm. andthe contact length of 2.0 cm., the film surfaces being in contact witheach other. A satisfactory method for obtaining improved biaxially drawnpolyamide films having a low friction coefficient has not beenpreviously reported. Biaxially drawn polyamide films having low frictioncoefficients have been obtained by the present invention and theimproved polyamide films have a friction coefiicient of less than three.

The following examples are given by way of illustration only:

Example 1 Substantially amorphous films of poly-e-caproamidc (relativeviscosity 3.0 at 25 C. in 96% sulphuric acid) having an averagethickness of 200p are pretreated with water to give the films a watercontent of about 3% (measured by Karl Fischers method) and density of1.130. Samples of the films were preheated to 70 C., 80 C., 100 C., 1100., 140 C. and 160 C. respectively by passing them through a preheatingzone, in which the atmosphere was maintained at 170 C. for a suitabletime e.g. 3 to 6 seconds. The preheated films were then simultaneouslydrawn in the longitudinal and transverse directions with a draw speed ofabout 24,000 percent min. for about 2.2 seconds by passing through adrawing zone about 0.9 m. in length, where the atmospheres wererespectively maintained at 140 C., 130 C., 120 C., 115 C., 160 C. and180 C. to draw the films with a magnification of about 3 in both thelongitudinal and transverse directions at a ratio of draw speeds ofabout 1-1.2. Two films drawn as described above and designated C wasprepared for comparison purposes except that the temperature of thepreheated films was 100 C. and the temperature of the atmosphere indrawing zone 80 C. The temperatures of the films were measured by usinga Radiation Pyrometer and the atmospheric temperatures. measured 10 mm.above the films, were thermocouples. The static friction coefficient ofthe film (average thickness about 22 obtained under the conditions aboveare shown in Table 1.

Table 1 illustrates the low static friction coefficient of filmsprepared by the present-process as compared with film wherein the drawtemperature was lower than the preheat temperature.

Substantially amorphous films (average thickness 160a; density1.128) ofpoly-e-caproamide (relative viscosity 3.0 measured at 25 C. in 96%sulphuric acid) were pretreated with water to give the films a watercontent of about 1.2, 3, 4.5 and 6.0% respectively.

The films were preheated to 70 C. by passing through a preheating zone,where the atmosphere was maintained at 130 C. for a residence time ofabout 4 seconds. The preheated films were then simultaneously drawn inthe longitudinal and transverse directions with a draw speed of about30,000 percent/min. in a drawing zone, where the atmosphere wasmaintained at 140 C., to draw the films with a magnification of 3 in thelongitudinal direction and 3.5 in the transverse direction at a ratio ofdraw speeds of about 0.6 to 1.0. I

For comparison purposes, two biaxially drawn films designated C and Cwere prepared under similar conditions to those described above exceptthat the water content of the films was about 0.5% and 8.0%respectively.

The static friction coeflicient of the film (average thickness about15,44) obtained under the conditions described above are shown in Table2 which illustrates the significance of the water content in thepractice of the present invention.

-Poly-s-caproamide (relative viscosity 2.7 at 25 C. in 96% sulphuricacid) was extruded by means of a conventional screw extruder having adiameter of 45 mm. to give a raw film having a thickness of 200 microns.The raw film was left in a vessel, in which the atmosphere was kept at20 C. and the relative humidity was 65%, to provide a film with a watercontent of about 3% (measured by Karl Fischers method). The film waspreheated to 'C. in an atmosphere having a temperature of 130 C. byconventional infra-red heaters for a period of 4 seconds.

The preheated film was then simultaneously drawn in the longitudinal andtransverse directions with a draw speed of about 24,000 percent/min. inan atmosphere having a temperature of 150 C. to provide a film having amagnification of about 3 in both the longitudinal and transversedirection at a ratio of draw speeds of 1 to 1.2. The drawn film washeat-set by heating for 10 seconds at 190 C. under tension so that itstransverse dimensions would be kept constant to give a film 22 inthickness. Microscopic observation of the surface of a drawn film showedthat throughout the surface numerous fine uneven Webs (of a height ofabout l-2 microns) were formed thereon. The static friction coeflicientof the drawn film measured by this method was about 1.0, and the filmobtained had improved antiblocking characteristics and its machinabilitywas improved. The film transparency measured at 450 m was 80%.

Another raw film was preheated and drawn for comparison purpose in thesame manner as described above except that the raw film had a watercontent of about 8%. The drawn film in comparison with that of this example had an even surface and various practical disadvantages, forexample, blocking, bad machinability, etc.

Example 4 A substantially amorphous film (density 1.030; thickness 2001.) of poly-1l-amino-undecanamide (relative viscosity 2.8 at 25 C. in96% sulphuric acid) was left in a vessel, in which the atmosphere waskept at a temperature of 35 C. and a relative humidity of to give a filmhaving a water content of about 1.2% (measured by Karl Fischers method).

The film was pre-heated to about C. by conventional infra-red heaters inan atmosphere maintained at C. for a period of 2.7 seconds. Thepreheated film Was then simultaneously drawn in the longitudinal andtransverse directions with a draw speed of Example A substantiallyamorphous film (density 1.125; average thickness 150g) ofpoly-e-caproamide (relative viscosity 3.2 at 25 C. in 96% sulphuricacid) was extruded at a speed of m./min. by means of a conventional T-die process using a 90 mm. extruder, and immediately immersed in a waterbath, maintained at 40 C. This provided an average water content for thefilm of about 3%. The film was then passed through a vessel, in whichthe atmosphere was maintained at a temperature of 50 C. and a relativelyhumidity of 60%.

The film was then immediately pre-heated to about 75 C. by passing itthrough a pre-heating zone 1 m. in length at a through-put speed of 10m./min. where the atmosphere was maintained at 90 C. by conventionalinfra-red heaters. The pre-heated film was then simultaneously drawn inthe longitudinal and transverse directions with a draw speed of about24,000 percent/ min. through a drawing zone, where the atmosphere wasmaintained at 150 C. to draw a film with magnification of about 3 in thelongitudinal direction and 3,4 in the transverse direction at a ratio ofdraw speeds of 0.6 to 1.0.

The drawn film was heat-set for 10 seconds at 190 C. under tension, inorder that its dimensions would be kept constant in a transversedirection, to give a film 1. in thickness. The polyamide film obtainedhad unevenly reticulated projections on its surface, and a staticfriction coefficient of 0.7. It had improved antiblockingcharacteristics and processability.

Example 6 Poly-e-caproamide (relative viscosity 3.0 a C. in 96%sulphuric acid) was extruded with a conventional screw extruder having adiameter of 65 mm. to give a raw film density1.l28having a thickness of215 microns. The raw film was dipped into a water bath, which wasmaintained at C. to provide an average water content of the film ofabout 3%. The film was then passed through a vessel, in which theatmosphere was maintained at a temperature of C. and a relative humidityof. and then pre-heated to C. by passing it through a preheating zone 1m. in length at a through-put speed of 10 m./min. the atmosphere beingmaintained at C. by conventional infra-red heaters. The preheated filmwas immediately drawn simultaneously in the longitudinal and transversedirections with a draw speed of about 30,000 percent/min. through adrawing zone, where the atmosphere was maintained at 120 C. byconventional hot air heaters, to draw a film having a magnification ofabout 3 in both the longitudinal and transverse directions at a ratio ofdra-w speeds of 1 to 1.2. The drawn film was heat-set for 10 seconds at200 C. under tension in order that its dimension in the transversedirection be kept constant to give a film 24 in thickness. The polyamidefilm obtained had unevenly reticulated fine projections on its surface,and a static friction coefiicient of 1.0 with improved antiblockingcharacteristics and processability.

Example 7 A substantially amorphous film (density 1.128; averagethickness of poly-e-caproamide (relative viscosity 3.0 at 25 C. in 96%sulphuric acid) was dipped into a water bath maintained at 35 C. to givea film having an average water content of about 2.5%. The film waspre-heated to about 90 C. for approximately 5 seconds in an atmosphereat 120 C. using conventional infra-red heaters in order to provide alarge difference of water content between the surface and the interiorof the film.

The pre-heated film was immediately drawn simultaneously in thelongitudinal and transverse directions with a draw speed of about 30,000percent/min. through a drawing zone, in which the atmosphere wasmaintained at 120 C. by conventional hot air heaters, to draw a film Cirhaving a magnification of about 3 in both the longitudinal andtransverse directions at a ratio of draw speeds of 0.9 to 1.2. The drawfilm was heat-set for 8 seconds at 200 C. under tension in order thatits dimensions would be kept constant in a transverse direction toprovide a film 15 in thickness.

The static friction coefiicient of the polyamide film was 1.2 and it hadimproved antiblocking characteristics and processability.

Example 8 A substantially amorphous film of polyhexamethylene sebacamide(average thicknessdensityl,057) was pretreated with water to give thefilm a water content of about 3%. The film was preheated to 130 C. bypassing through a preheating zone, where the atmosphere was maintainedat 180 C. The preheated film was then simultaneously and biaxially drawnin the longitudinal and transverse directions with a draw speed of about30,000 percent/min. in a drawing zone, where the atmosphere wasmaintained at 140 C. to draw the film with a magnification of 3 in thelongitudinal direction and 3.5 in the transverse direction at a ratio ofdraw speeds of about 0.6 to 1.0. The drawn film was heat-set by heatingfor 10 sec. at C. under tension 50 that its transverse dimension wouldbe kept constant whereby to yield a film having a thickness of 15,. Thestatic friction coefficient of the drawn film was about 2.5. It wasobserved that the obtained film had improved antiblockingcharacteristics and machinability.

Various modifications can be made to the instant invention. For example,a suitable additive may be added to the water absorbed by the film so asto enhance its heat transmision without adverse effect on the filmproperties.

Having described the present invention that which is sought to beprotected is set forth in the following claims.

What is claimed is:

1. A process for drawing a polyamide film which comprises:

(a) treating the polyamide film with water so that it contains 1 to 6%of water by weight (based upon the weight of the film);

(b) preheating said film for 0.5 to 30 seconds to a temperature which is70 to 35 C. below the melting point of the film so as to impart asubstantial difference in water content between the surface and interiorof the film, and

(c) drawing the film simultaneously in the transverse and longitudinaldirections at a temperature at least 5 C. higher than said preheattemperature but not higher than 30 C. below the melting point of thefilm at a draw ratio from 16:1 to 4:1.

2. The process of claim 1 wherein the polyamide film is selected from amember of the group consisting of poly-e-caproamide,poly-hexamethylene-adipamide, polyhexamethylene sebacamide,poly-l-aminoundecanamide, poly-laurinamide, a copolymerised polyamide ora mixture of two or more thereof.

3. A process as claimed in claim 1 in which the said film is a film ofpoly-hexamethylene-sebacamide and from 1 to 5% of water by weight (basedupon the weight of the film) is absorbed.

4. A process as claimed in claim 1 in which the said film is a film ofpoly-1l-aminoundecanamide and from 1 to 3% of water by weight (basedupon the weight of the film) is absorbed.

5. The process of claim 1 wherein the polyamide film passes through azone with an atmosphere at a temperature at least 5 C. above thetemperature of the preheated film before drawing to a point where theratio of average thickness of the drawn film to average thickness of theundrawn film is 0.5: l.

6. The process as claimed in claim 1 in which the said 9 film is drawnat a speed of from 6000 to 100,000 percent/ IIllIl.

7. A process as claimed in claim 1 in which the ratio of thelongitudinal and transverse draw speeds is from 2:1 to 0.5: 1.

8. The process of claim 1 wherein the film is drawn at a speed of 6000to 100,000 percent/min, the longitudinal and transverse draw speedsrange from 2:1 to 05:1 and the draw ratios are from 16:1 to 4:1.

9. The process of claim 1 wherein the film is preheated for a period of3 to 6 seconds.

10. A process as claimed in claim 1 in which the drawn film is heat-setat a temperature not higher than 10 C. below the melting point of thefilm with the film maintained under tension such that the transversedirection remains substantially constant.

References Cited UNITED STATES PATENTS 6/ 1966 Helfelfinger 264-2896/1966- Hetfelfinger 264289 1/1961 Long s 264289 6/1951 Pace 2642897/1944 Markwood 26078 5/1939 Miles 26078 5/1970 Tsuruta, et a1 264289US. Cl. X.R.

